1. Field of the Invention
The present invention relates to a fuel supply control system for an internal combustion engine of the type equipped with a device for purifying exhaust gases such as a catalyst or reactor, which is effective in preventing overheating of the exhaust purifying device during high speed and high load operations of the engine.
2.
Description of the Prior Art
In a known type of electronically controlled fuel injection system in which the amount of air drawn into an engine and the rotational speed of the engine are electrically detected to control the amount of fuel supplied to the engine, a throttle switch is usually provided to detect the opening of the throttle valve, whereby when the throttle opening exceeds a predetermined opening (hereinafter referred to as a wide open enrichment opening), in response to the output of the throttle switch the amount of fuel supplied to the engine is increased to enrich the mixture drawn into the engine. This is done with the intention of increasing the maximum power output of the engine during periods of high load operation. However, where there is no secondary air supply, the temperature of an exhaust purifying device, e.g., a catalyst or reactor for purifying the exhaust gases from the engine is at its maximum when the air-fuel ratio of the mixture is at around the stoichiometric ratio, and when the mixture is rich, the temperature decreases as the mixture is enriched but when the mixture is leaned, the temperature does not decrease so much as the mixture is enriched. Moreover, with known exhaust purifying devices of the type which are not supplied with secondary air, the temperature of such exhaust purifying device generally tends to become higher than the permissible temperature before the throttle valve is opened to the wide open enrichment opening during high speed and high load operations of the engine. To overcome this difficulty, a method has been proposed in which the wide open enrichment opening for the throttle switch is preset small, thus increasing the engine operating ranges where the mixture is enriched and thereby preventing overheating of such exhaust purifying device during periods of high speed and load operations where the temperature of the device tends to increase. With this method, however, any excessively small setting of the wide open enrichment opening tends to cause excessive exhaust gas emissions during normal running conditions, and there is thus a lower limit to the wide open enrichment opening which in turn makes it impossible to effectively prevent overheating of the exhaust purifying device.